Wanwan, there were many errors in your code fragment,
but I think that the heart of the problem was a failure
to understand that \myfont is a parameterless macro.
The following shews how \myfont can be successfully
used, and if you want a parameterised version,
\MyFont is an example.
Philip Taylor
--------
\documentclass {minimal}
%\ProvidesPackage{mypackage}
\font \tt = cmtt10
\newfont{\myfont}{cmss10}
\def \MyFont #1{{\myfont #1}}
\renewcommand{\AA}{{\char221}}
\newcommand{\AB}{{\char222}}
\newcommand{\AC}{{\char223}}
\begin {document}
and I using it in my tex
%\usepackage{mypackage}
my question is how can I use command like
\begingroup \myfont a\AA b\AB c\AC \endgroup
Using command \begin{myfont} a\AA b\AB c\AC
\end{myfont} is working well but not {\tt \string \newfont} ?
Here is {\tt \string \MyFont} in use :
\MyFont {a\AA b\AB c\AC}
anybody pls
\end {document}